The marine Indo-Pacific cowry, Cypraea caputserpentis, shows geographic variation of life history and shell morphology. Adult body size increases with latitude, but shell thickness decreases with latitude. To explain the clinal variations, we study a mathematical model of the optimal growth schedule. The life history of cowries consists of three stages: shell volume increases in the juvenile stage, which is followed by the callus-building stage in which shell thickness increases, and then reproduction starts without further growth in the adult stage. We calculate the lengths of juvenile and callus-building stages that maximize lifetime reproduction. By considering latitudinal change in the mortality and growth-promoting factors, the observed clinal patterns of juvenile traits can be explained by a negative latitudinal gradient of shell-crushing predators. This suggests the importance of a latitudinal gradient of predation pressure for body-size clines in marine ectotherms. On the other hand, latitudinal clines of shell thickness can be explained by a latitudinal gradient of either shell-crushing predation pressure or one of the environmental factors promoting shell thickening, such as seawater temperature.
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